1. Field of the Invention
The present invention relates in general to thermal inkjet printers, and more particularly to methods and apparatus for determining the appropriate electrical firing parameters for driving a printhead to reliably eject ink droplets
2. Description of the Related Art
The following details a technique for in-printer tuning of the fire pulse width using thermal nucleation detection. For a given ink formulation and ejector design, it is desirable that a precise dosage of energy be delivered for proper ink bubble nucleation and droplet ejection. The amount of energy applied to the nozzle heater of a printhead is adjusted by modifying the pulse width that is applied during an ejection event. If the amount of thermal energy applied to the ink is too low, then the ink jet will be weak, leading to low droplet velocity and increased droplet misdirection. If the energy is too high, this will lead to higher temperature variation and flooding related print defects, as well as decreased heater life. The ability to tune the firing pulse width reduces variations in printer/printhead combinations which results from normal manufacturing variations. The tuning of a printhead in a printer leads to more consistent droplet formation and droplet mass, and less droplet misdirection over a population of different printheads used in the printer.
Printer manufacturers have provided the capability of in-printer fire pulse width tuning using a drop velocity measurement method. This method uses an optical alignment sensor to scan printed patterns and measure the drop velocity for a set of fire pulse widths. The measured velocities are then used to select an operating fire pulse width corresponding to a droplet velocity above the “knee” of a jetting curve.
Another method of tuning a printhead involves printing a continuous purge bar while reducing the fire pulse width. Then, the pattern is scanned with an optical sensor and the print density is evaluated to determine the operating fire pulse width.
The operating energy for driving the printhead of a thermal inkjet printer can be determined by other means. U.S. Pat. Nos. 5,418,558; 5,428,376 and 6,820,958 disclose other methods for determining the optimum drive signal for a operating a printhead.
From the foregoing, it can be seen that a need exists for a method of determining a printhead nucleation point that can be used to select an appropriate fire pulse width without the need for printing a sample and without the need for external sensors. Another need exists for a method of determining the nucleation point using thermal feedback from the thermal sense resistors of the printhead.